Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells. The direction of the static magnetic field B0 is commonly denoted as the z-axis, while the plane perpendicular to the z-axis is commonly termed the x-y or θ-plane.
The frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0. In order to maximize the accuracy of NMR measurements, the resonant frequency of the excitation/detection circuitry is set to be equal to the frequency of interest. The resonant frequency of the excitation/detection circuitry is generallyv=√{square root over (LC)}  [1]where L and C are the effective inductance and capacitance, respectively, of the excitation/detection circuitry.
Additionally, in order to maximize the transfer of RF energy into the RF coils, the impedance of each coil is matched to the impedance of the transmission line and associated components used to couple RF energy into the coil. If the coil is not impedance-matched, a sub-optimal fraction of the RF energy sent to the coil actually enters the coil. The rest of the energy is reflected out, and does not contribute to the NMR measurements.
Several approaches have been proposed for adjusting the circuitry of NMR spectrometers to achieve desired resonance frequency tuning or impedance matching. For example, in U.S. Pat. No. 6,204,665, Triebe et al. describe an NMR probe including movable adjustment rods for tuning the resonant frequency of the probe resonator. The probe also includes an actuator for performing remotely-controlled adjustments of electrical and or mechanical units such as variable resistors, inductors, and trimmer capacitors. In U.S. Pat. No. 5,986,455, Magnuson describes a tuning apparatus including a plurality of capacitors which can be switched into a tuning circuit by controllable switches. In U.S. Pat. No. 5,081,418, Hayes et al. describe a method of tuning a radio-frequency coil by connecting sections of the coil to ground.